Video: Optical Telescope: Facts & Overview

This lesson will explain what optical telescopes are, their history, how they work, and how they're used. We will also review the three types of telescopes, including the kinds of aberration (distortion) from which they can suffer. A short quiz will follow.

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what optical telescopes are, their history, how they work, and how they're used. We will also review the three types of telescopes, including the kinds of aberration (distortion) from which they can suffer. A short quiz will follow.

What Is an Optical Telescope?

For thousands of years, humans have gazed up at the stars and wondered what's out there. Throughout most of human history, the question was mostly left to philosophers and theologians. It was only with the invention of the optical telescope that we could finally take a closer look and begin to answer our many questions about the universe.

A telescope is a device that gathers light (or electromagnetic waves) and focuses it to form an image. An optical telescope is a telescope that gathers light in the visible (or optical) part of the electromagnetic spectrum.

The Electromagnetic Spectrum

Electromagnetic waves (or electromagnetic radiation) are waves made of oscillating magnetic and electric fields. The electromagnetic spectrum includes visible light, radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays. Modern astronomers have telescopes that take pictures in all parts of the electromagnetic spectrum: optical telescopes, radio telescopes, x-ray telescopes. . . but hundreds of years ago, we couldn't do that.

The First Telescope

The way that lenses could magnify or sharpen images had been understood since the days of the Ancient Greeks. But it was only when eyeglasses were made in Western Europe from the 13th century onwards, that the quality of lenses started to improve. And as the quality of lenses improved, the possibility of using them to look at the stars began to occur to people.

The first known optical telescope to be built was in the Netherlands in 1608. Eyeglass makers Hans Lippershey and Zacharias Janssen worked with optician Jacob Metius to build a telescope. Though their images were blurry, it was (as far as we know) the first of its kind.

Just a year later, Galileo greatly improved upon the design and was the first to use telescopes for astronomy. The images still suffered from several types of aberration (distortion), but it was a huge step forward.

Three Types of Optical Telescopes

There are three main types of optical telescopes: refracting, reflecting, and catadioptric. Pure refracting telescopes are the rarest kind in modern times, but there are examples of all three kinds of telescopes in use today.

Refracting telescopes use a combination of lenses to focus light onto an eye-piece, photographic plate, or sensor. The most basic kinds of refracting telescopes, such as the ones Galileo built, suffer from a kind of distortion called chromatic aberration. This is where light of different colors bends by different amounts, causing it to focus at different points, blurring the image. This can be fixed in modern telescopes using multiple lenses.

Reflecting telescopes take advantage of the fact that shaped mirrors produce results very similar to lenses. This idea was understood since the days of the Arabic scientist Alhazan in the 10th and 11th centuries, but when Galileo and Newton considered the possibility, they were still incredibly difficult to manufacture. Newton was one of the first to create a working reflecting telescope in 1668, but due to the difficulty in getting the mirrored surface the correct shape, the image quality was not very good.

Reflecting telescopes suffer from another kind of distortion called spherical aberration, which is where beams of light from different places focus at different points. This happens because the surface is spherical, hence the name. Though it can be challenging, this aberration can be fixed by adjusting the mirror to a perfect parabolic shape.

Catadioptric telescopes use a mixture of lenses and mirrors to maximize the light collection and minimize the distortion of the telescope.

Lesson Summary

An optical telescope gathers light (electromagnetic waves in the visible, or optical, part of the spectrum) and focuses it to form an image. Astronomers use telescopes that cover the whole of the electromagnetic spectrum, but the first telescopes were purely optical telescopes. Galileo was the first known scientist to use a telescope for astronomy; before his time, our ability to manufacture high quality lenses was not good enough to create such a telescope.

There are three kinds of telescope: refracting, reflecting, and catadioptric. Refracting telescopes use lenses to focus light, reflecting telescopes use curved mirrors, and catadioptric telescopes use a mixture of both.

Refracting telescopes can suffer from chromatic aberration, and reflecting telescopes can suffer from spherical aberration. In both cases, it makes the image blurry. Chromatic aberration can be corrected by using multiple lenses, and spherical aberration can be corrected by using a parabolic mirror.

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